Is this a 'trivial-sounding matter'?

is he violent or vindictive?

It doesn't matter.

As it is, I don't know anything about the two parties involved - but what do you think about that comment?

I think he is being intimidating, but has plausible deniability ie, it may cause your family financial pain. Sounds a lovely person

It depends on who it is who sent it, they could claim they mean "pain" from a financial perspective or something else.

Tricky one.

Thin veiled threat - court would be interested by it's entry as evidence

Surely it doesn't matter why it was sent. Is it a "trivial matter"?

It's fantastic ammunition for the receiver of the email.

Yes, it's trivial. The sender is panicking. It's obviously not a physical threat. Who would be mug enough to threaten someones family by email?